The Use of False Acoustic Signatures by Submarines: Perfidy or Ruse of War?

[Pornomo Rovan Astri Yoga is a PhD candidate in International Law at the Australian National Centre for Ocean Resources and Security (ANCORS), University of Wollongong, and an officer in the Indonesian Navy (TNI AL). He holds a master’s degree in Information Strategy and Political Warfare from the US Naval Postgraduate School (NPS) and has over a decade of operational experience aboard warships]    

In recent years, naval analysts have pointed to the increasing sophistication of underwater deception. Modern navies now experiment with decoy systems capable of mimicking the unique acoustic signatures of submarines. The United States, for instance, has deployed expendable mobile acoustic training targets (EMATTs) and the AN/SLQ-25 “Nixie” system, designed to lure torpedoes by imitating submarine sounds. Russia reportedly possesses similar technology used to mislead or complicate enemy sonar detection.

This is not just a technical curiosity—it raises a novel legal question. If a submarine can “sound like” another submarine—whether an enemy or even a neutral one—what does international humanitarian law (IHL) say? Are we looking at a clever ruse of war, or does this cross the line into perfidy?

The Concept of Perfidy and Ruses of War

The law of armed conflict rests on the principle of distinction. Combatants and military objectives may be attacked; civilians and civilian objects may not. To give this principle practical effect, international law has long required that combatants distinguish themselves. That obligation takes many forms: wearing uniforms, carrying arms openly, using national flags and ensigns, and respecting distinctive signals such as the Red Cross. The aim is to ensure that enemies can tell who is a lawful target and who is protected.

Yet deception has always been a central feature of warfare. From camouflage and dummy tanks to stealth aircraft and electronic countermeasures, militaries routinely mislead the adversary. International humanitarian law distinguishes between lawful ruses of war and unlawful perfidy. Article 37 of Additional Protocol I defines perfidy as acts that invite the enemy’s confidence in a protected status under the law of armed conflict, with the intent to betray that trust. Ruses, by contrast, are acts intended to mislead or induce mistakes without violating any legal rule.

The distinction is conceptually clear but practically blurry. Some ruses are lawful; others are prohibited. A false surrender using a white flag is perfidious. Camouflage or spreading false intelligence is a lawful ruse. But as every practitioner knows, many forms of deception live in the gray zone—especially at sea, where the law of naval warfare evolved along a different path.

Naval Warfare’s Unique Approach to Perfidy and Ruses

The sea has always demanded a different legal logic. On land, private property cannot be seized. At sea, enemy merchant ships—though privately owned—could lawfully be captured as prize. On land, combatants cannot wear the enemy’s uniform or neutral flags. At sea, warships may fly false colors, provided they show their true flag before opening fire.

This divergence arose from the environment. On land, terrain provides natural cover. At sea, ships are exposed. To balance that vulnerability, naval custom tolerated certain deceptive practices. During the age of sail, and even through the World Wars, ships routinely used false flags or disguised themselves as merchantmen—so-called Q-ships. These tactics were lawful so long as the true flag was hoisted before an attack.

Modern technology has eroded the utility of such methods. Radar, satellites, and electronic signatures reveal far more than flags do. Identification now often depends on emissions, not ensigns. Still, the core rule endures: deception at sea is broadly tolerated, but perfidy—exploiting a protected status like that of a hospital ship—remains prohibited.

The Submarine Exception

Submarines occupy an ambiguous place in this framework. By design, they are invisible. Their defining feature—submergence—provides a tactical advantage akin to camouflage. A submerged submarine cannot display flags or visual signals; deception through ensigns is meaningless below the surface.

This sets them apart not only from surface warships but also from military aircraft. Aircraft have no recognized right to use false flags or feign civilian status even when they fight above the sea. A warplane cannot disguise itself as a commercial airliner. Yet submarines, even when submerged, are already shrouded in deception by nature.

The question is whether additional layers of submarine deception—particularly false acoustic signatures—should be treated as lawful ruses or as perfidy.

Acoustic Signatures as the “Flags” of Submarines

Every submarine has a unique “acoustic fingerprint.” Its engines, machinery, and propellers produce a pattern of sound that naval intelligence agencies catalog and use to identify vessels. Just as a flag signals nationality on the surface, an acoustic signature signals identity underwater.

Traditionally, submarines sought to minimize their noise to evade detection. But advances in underwater drones and acoustic technology now make it possible to mimic another submarine’s signature. A vessel could project the sound of a different submarine—or use a decoy unmanned underwater vehicle (UUV) to create confusion.

This raises a crucial legal dilemma: if a submarine uses a false acoustic signature, is it equivalent to flying a false flag? And if so, must it “hoist its true acoustic flag” before launching a torpedo?

False Flags, Then and Now

Looking back helps frame the issue. In the sailing era, a false flag worked because ships closed to visual range before firing. Deception was effective until the last moment, but international law required that the attacker show its true colors before battle. In the World Wars, Q-ships went further, disguising their very structure. They revealed their identity only at close quarters, when U-boats surfaced to engage.

Modern long-range missile warfare complicates this logic. When a warship fires from beyond the horizon, simply hoisting an ensign is inadequate. The target cannot see it. As naval operations rely increasingly on electronic signatures, the question has shifted: must a warship not only lower its false flag but also transmit truthful data before striking?

This dilemma directly informs the submarine case. If acoustic signatures are the underwater analogue of flags, then deception by mimicry may fall under the same rule: lawful until the moment of attack, at which point the submarine must reveal its true identity.

Submarine Deception and Perfidy

Would false acoustic signatures ever amount to perfidy? The answer depends on what identity is mimicked. If a submarine projects the sound of a hospital ship or a vessel entitled to special protection, that would clearly be perfidious. But submarines are never hospital ships; they are almost always combat vesselswith some exceptions of very small research submarines.

The more realistic danger is a submarine projecting the signature of a neutral vessel. That tactic resembles flying a neutral flag. At sea, neutral flags may be used until the moment of attack, when they must be replaced with the true flag. By analogy, false acoustic signatures imitating neutral submarines may be tolerated until combat begins.

But unlike flags, acoustic deception is harder to reverse. A flag can be raised in seconds. Altering a submarine’s acoustic signature may require shutting down systems, changing speed, or even using external devices like UUVs. This means that “revealing true identity” underwater may not be practically possible at the moment of attack. The law has yet to catch up with this technological reality.

Unmanned Underwater Vehicles and the Future of Deception

The introduction of UUVs deepens the puzzle. Navies already use UUVs to simulate submarine signatures during training. In conflict, UUVs could act as decoys, drawing enemy fire or creating the illusion of multiple submarines.

If a UUV projects the signature of a neutral submarine, the parent navy effectively engages in an “underwater false flag.” Is that lawful? The analogy to surface ships suggests yes, until the moment of attack. But again, without state practice or treaty law, the legality is unsettled.

Moreover, deception by UUVs creates risks of misidentification. A surface ship might believe that the identified object is a neutral vessel and therefore, makes a surface ship hesitate to act. Or worse, it might mistakenly attack a real neutral submarine, triggering diplomatic crises. These dangers mirror those posed by false flags but amplified by the invisibility of the underwater domain.

Aircraft, Warships, and Submarines: A Comparative Lens

The sharp contrast with aircraft highlights the legal gap. Military aircraft are forbidden from pretending to be civilian planes. There is no equivalent of the naval false flag. Submarines, however, exist in a different realm. They are inherently deceptive but interact primarily with other combatants. They do not share sea lanes with “civilian submarines.” That difference may explain why international law has not yet squarely prohibited false acoustic signatures. Yet it also suggests that submarines require a specific regulatory framework, just as aircraft have theirs.

Should There Be an Obligation to Reveal an “Acoustic Flag”?

This brings us back to the central question: must a submarine reveal its true acoustic signature before launching an attack, just as surface ships must raise their true flag?

The logic of distinction suggests yes. If acoustic signatures serve as the functional equivalent of flags, then international law should demand clarity before hostile action. Otherwise, adversaries cannot reliably distinguish between lawful and unlawful targets.

Conclusion

Submarines operate in a domain uniquely suited to deception. Their acoustic signatures function as flags, and false acoustic profiles are a modern ruse of war. Deception becomes perfidy only when a vessel imitates a protected status, which submarines do not. The key legal question is whether the “reveal” requirement—analogous to hoisting a true flag—applies to acoustic deception. Technology, tactical realities, and the absence of state practice leave this issue unresolved. For now, the silence of the law mirrors the silence of the sea. Sound may be the new flag, but the rules of how it must be shown remain unwritten.